Comprehensive and Detailed Explanation From Exact Extract:

Axial resolution improves with shorter spatial pulse length, which is directly related to higher frequency. At shallow depths (such as 2 cm), higher frequency can be used effectively since attenuation is minimal.

Principles and Instrumentation state:

'Axial resolution improves with increasing frequency due to shorter wavelength and pulse length.'

Turning off spatial compounding (A) affects speckle reduction.

Harmonics (B) help with resolution but primarily lateral contrast.

Gain (C) affects brightness, not resolution directly.

Therefore, the correct answer is D: Increase frequency.